Conventional turbochargers are driven by waste exhaust heat and gases, which are forced through an exhaust turbine housing onto a turbine wheel. The turbine wheel is connected by a common turbo-shaft to a compressor wheel. As the exhaust gases hit the turbine wheel, both wheels rotate simultaneously. Rotation of the compressor wheel draws air in through a compressor housing, which forces compressed air into the engine cylinder to achieve improved engine performance and fuel efficiency. Turbochargers for variable speed/load applications are typically sized for maximum efficiency at torque peak speed in order to develop sufficient boost to reach peak torque. However, at lower speeds, the turbocharger produces inadequate boost for proper engine transient response. Conversely, the turbocharger produces too much boost at rated speed and load. As a result, a wastegate is often used, which allows the use of a smaller turbocharger to reduce lag while preventing it from spinning too quickly at high engine speeds.
The wastegate is a valve that allows the exhaust to bypass the turbine blades. The wastegate is used to control the boost pressure. If the pressure gets too high, it could be an indicator that the turbine is spinning too quickly, so the wastegate bypasses some of the exhaust around the turbine blades, allowing the blades to slow down. This wasted energy reduces overall engine efficiency but prevents damage to the turbocharger from over-speed and prevents damage to the engine from over boosting.
Industry has recognized the wasted energy and has made attempts to harness the wasted energy. For example, U.S. Pat. No. 6,553,764 discloses a turbocharger system that mechanically couples a first motor/generator to the turbo-shaft of the turbocharger system wherein the first motor/generator is coupled to a second motor/generator that is coupled to a flywheel. During periods of excess turbocharger boost, the turbo-shaft drives the first motor/generator as a generator to provide power to drive the second motor/generator as a motor and store energy in the flywheel. During periods of insufficient boost, the energy stored in the flywheel is used to drive the second motor/generator as a generator to drive the first motor/generator as a motor, which drives the turbo-shaft, to accelerate the turbo-shaft more quickly. While this system is more efficient than conventional systems, the use of a flywheel creates additional problems. These problems include the flywheel failing destructively and damaging other components as it breaks apart into shrapnel-sized pieces, added weight to contain the flywheel, decreased stability during turns, increased control complexity to counter the forces generated by the flywheel, etc.
Another approach is illustrated in U.S. Pat. No. 5,113,658 that operates during periods of insufficient boost uses a hydraulic assist turbine mounted on the turbo-shaft between the compressor and turbine. During operation when the turbocharger does not provide sufficient boost, pressurized hydraulic fluid is supplied as high energy jets to the hydraulic assist turbine for rotating the hydraulic assist turbine, which in turn, drives the turbo-shaft. At periods of sufficient boost and excess boost, pressurized hydraulic fluid is not supplied to the hydraulic assist turbine. While this approach is more efficient than conventional turbocharger systems, this approach does not solve the problem of harnessing the wasted energy during periods of excess boost.
Another approach is illustrated in U.S. Pat. No. 6,343,473. In this approach, a supercharger is put in series with a turbocharger. During operation when the turbocharger does not provide sufficient boost, the supercharger is used to provide compressed air to the turbocharger compressor. At operational points where there would normally be excessive boost, the amount of air provided to pressurize the inlet is reduced by diverting the air flow, thereby reducing the amount of compressed air fed to the turbocharger compressor inlet. The problem with this approach is that the turbocharger is undersized and smaller than normal due to the supercharger providing compressed air to the turbocharger compressor. Should the supercharger fail since it is in series with the turbocharger, the turbocharger on its own is insufficient to provide the low speed torque required for starting and accelerating performance.
A further approach is illustrated in U.S. Pat. No. 5,729,978. In this approach, a turbocharger is used with a mechanical step-up transmission connected to the turbo-shaft to increase the torque during low speed operation. The step-up transmission includes a step-up gear (i.e., a two-stage change-speed gearbox) and a controllable hydrostatic coupling. To achieve shorter response times during transient operation, the hydrostatic coupling is locked up by a mechanical or electro mechanical clutch. The system decouples the exhaust gas turbine from the compressor at low rotational speeds so that the mechanical gearbox only has to drive the compressor. At higher speeds, the turbine is accelerated by exhaust gas flow and at a specified speed, it is coupled onto the turbocharger shaft by the clutch. The problems introduced with this system include the use of further components such as the step-up gear and the hydrostatic coupling, which reduces overall reliability. In the event that the hydrostatic coupling fails, the turbocharger operation may fail.
What is still needed is a system the increases efficiency that does not have the above-mentioned problems. The invention provides such a system. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.